Regulations for preventing the escape of volatile hydrocarbon fuel vapors to the atmosphere are being promulgated by governmental agencies. One source of hydrocarbon fuel vapors is the fuel tanks of vehicles using gasoline or other hydrocarbon fuels of high volatility. With current automotive fuel tank designs, fuel vapor can escape during the filling of the tanks and usually even after the tank is filled.
The use of an on-board vapor recovery system to remove excess fuel vapor from the fuel tank is one solution to this problem. Typically a canister with activated charcoal therein receives fuel vapors through a control unit mounted in the top of the fuel tank and communicates with the intake manifold of the vehicle engine for exhausting fuel vapor from the canister during operation of the engine. The control unit usually has a valve responsive to the level of fuel in the tank that enables the valve to stay open at a sufficiently low fuel level to permit fuel vapors to flow freely from the tank into the canister. As the fuel level rises during filling to approach a desired maximum level of fuel in the tank, a float is raised to close the valve to prevent liquid fuel from flowing through the valve and into the vapor receiving canister.
Some of these systems use a high capacity or high flow rate valve to control the flow of vapor from the fuel tank to the vapor storage canister. Current high capacity or high flow rate valve designs tend to be forced into and held in a closed position, when they should be open, by the vapor pressure in the fuel tank. This prevents the vapor from flowing through the valve and into the vapor storage canister. Thus, the purpose of the system is defeated because the vapor is not allowed to pass through the valve and into the storage canister, and then may be even discharged to the atmosphere.